A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. The light source can be UV, DUV, EUV-light or an electron beam.
In the lithographic apparatus as hereabove described, a common problem is how to create a stable and conditioned environment for the position measuring devices that are used to locate and position the stage relative to the base. Such a contact-less measurement device is typically an interferometer, however, other devices such as optical or capacitive rulers (encoders) can also be applied. Here, the stage is used, for example, for holding a target substrate thereon; in order to illuminate the target substrate in synchronicity with a movable patterning device, the substrate is moved in a scan direction, and the precise positioning and alignment relative to the patterning device usually takes place in a closed loop control in which the position of the stage is measured by interferometric measurement and adapted in response to the measurement. Oftentimes, position measurement devices are used that are very sensitive for variations of the media through which a position measurement is performed. For example, for an interferometer, a variation of the refractive index in the media is directly translated in a deviation of a measured position of the stage. Therefore, the media, and in particular the media in the volume that is present between the measuring device and the movable stage, should be kept stable. For an interferometer, the media should be kept at a stable and predictable variation of the refractive index, which is known to be sensitive to gas composition, pressure and temperature fluctuation. In general, the variation is preferably kept at a near-zero level.
In order to provide a stable media environment for letting a measurement beam pass through, a known concept is the so-called “air shower.” This is a gas stream of a conditioned, in particular, thermo-conditioned, pressure regulated and/or purified gas mixture that is ejected in the path that a measurement beam travels in the measurement setup. In the remainder, the term “air” will be used to indicate gas media that are used in the environment of the base and stage. This gas media may be ambient air, possibly conditioned and purified. However, also other gas media, such as pure nitrogen, may be included by this term and the invention is not restricted to air showers with ambient air.
It has been found that the conventional setup of a moving stage disturbs these air showers and more particularly, that there is a need to further improve the media environment conditions in the volume between the measuring device and the stage, in order to comply with the ever increasing demand for higher accuracy.